A large-sized columnar body typically used as a mast of a racing yacht is required to have light weight, high rigidity and high strength, for securing the high speed stability, maneuverability and safety of the boat. In addition, aerodynamically for the necessity of high air releasability, a large-sized columnar body with a simple circular cross section is insufficient, and a large-sized columnar body with an ellipsoidal cross section or with a specific streamlined cross section selection of which is based on wind tunnel experiments is demanded.
A long and large columnar body generally called a pole must be tapered for the necessity of weight balance in the longitudinal direction.
As large-sized columnar bodies to meet these demands, those made of FRP are attracting attention in recent years, and are proposed in (1) Japanese Patent Laid-Open (Kokai) No. 57-60989, (2) Japanese Patent Laid-Open (Kokai) No. 59-63289, (3) Japanese Patent Laid-Open (Kokai) No. 4-255306, etc.
(1) Japanese Patent Laid-Open (Kokai) No. 57-60989 discloses a technique to reinforce the rigidity of a mast of a sling boat by letting the mast have a reinforcing portion swelling outside at least in either bow or stern direction. Furthermore, the patent gazette discloses a structure in which layers oriented in directions of .+-.20 to .+-.70 degrees with respect to the longitudinal axis of the mast are formed inside the reinforcing portion, for the purpose of improving the torsional rigidity of the mast.
(2) Japanese Patent Laid-Open (Kokai) No. 59-63289 discloses a mast made of FRP for surfing with a hollow cylindrical structure in which an intermediate layer with glass fibers and reinforcing fibers with a modulus higher than that of glass fibers is held between layers with glass fibers as reinforcing fibers.
(3) Japanese Patent Laid-Open (Kokai) No. 4-255306 discloses a large-sized columnar body in which two shells prepared by laying up unidirectional prepreg sheets consisting of carbon fibers and an epoxy resin are bonded in the longitudinal direction to form an integral structure.
In the meantime, FRP is low in compressive strength as compared with tensile strength, unlike metallic materials, and for this reason, when FRP is used as a large-sized columnar body, how to highly manifest the compressive strength of FRP is a key issue to achieve weight reduction. In other words, in most cases, the compressive strength decides the weight of a large-sized columnar body.
While all of the above patent gazettes (1) to (3) relate to large-sized columnar bodies made of FRP with light weight and high performance, the patent gazette (3) only describes that it is desirable to let a layer of 0 degree in fiber orientation degree account for 50% or more or, as required, to lay up a 45-degree layer or 90-degree layer in combination, as a means for improving the compressive strength of a large-sized columnar body.
The object of the present invention is to provide a large-sized columnar body made of FRP improved in compressive strength, and with lighter weight and higher safety.